In general, a lithographic printing plate has a surface composed of an oleophilic image area and a hydrophilic non-image area. Lithographic printing is a printing method comprising supplying alternately dampening water and oily ink on the surface of lithographic printing plate, making the hydrophilic non-image area a dampening water-receptive area (ink unreceptive area) and depositing the oily ink only to the oleophilic image area by utilizing the nature of the dampening water and oily ink to repel with each other, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive layer (image-recording layer) has heretofore been broadly used. Ordinarily, a lithographic printing plate is obtained by conducting plate-making by a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then treating the exposed lithographic printing plate precursor to remove the photosensitive layer in the unnecessary non-image area by dissolving with a an alkaline developer or an organic solvent thereby revealing a surface of the hydrophilic support to form the non-image area while leaving the photosensitive layer in the image area.
In the hitherto known plate-making process of lithographic printing plate precursor, after the exposure, the step of removing the unnecessary photosensitive layer by dissolving, for example, with a developer is required. However, it is one of the subjects to simplify such an additional wet treatment as described above. As one means for the simplification, it has been desired to conduct the development with a nearly neutral aqueous solution or simply with water.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate technique of carrying the digitalized image information on highly converging radiation, for example, laser light and conducting scanning exposure of a lithographic printing plate precursor with the light thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of the important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
Based on the background described above, adaptation of plate-making operation to both simplification and digitalization has been demanded strongly more and more than ever before.
In response to such a demand, for instance, it is described in JP-A 2002-365789 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) that by incorporating a compound having an ethylene oxide chain into an image-forming layer of a lithographic printing plate precursor comprising a hydrophilic support and the image-forming layer containing a hydrophobic precursor, a hydrophilic resin and a light to heat converting agent, the lithographic printing plate precursor enables printing after conducting exposure and wet development processing using as a developer, water or an appropriate aqueous solution, besides on-machine development.
Also, a processing method of lithographic printing plate precursor is described in US2004/0013968A1 which comprises preparing a lithographic printing plate precursor comprising (i) a hydrophilic support and (ii) an oleophilic heat-sensitive layer which contains a radical-polymerizable ethylenically unsaturated monomer, a radical polymerization initiator and an infrared absorbing dye, is hardened with infrared laser exposure and is developable with an aqueous developer containing 60% by weight or more of water and having pH of 2.0 to 10.0, exposing imagewise the lithographic printing plate precursor with an infrared laser, and removing the unhardened region of the heat-sensitive layer with the aqueous developer.
On the other hand, methods of preparing a lithographic printing plate using a developer containing a sulfite are described in JP-A-2003-66624 (corresponding to US2003/0138732A1), JP-A-2003-287880 and JP-A-2004-109427. However, all these methods use an alkali developer having pH of 10 or more and there is no description as to combination capable of undergoing development with an aqueous developer having pH of 2.0 to 10.0. In addition, improvement in the developing property utilizing a reaction between a sulfite and a polymer is not suggested.